On the origin of speciation

Evolution abounds, and science shows how, but still explores why

Charles Darwin's contributions to biology evolved from lessons learned
and species collected during a five-year voyage (1831-36) aboard the HMS
Beagle. It was the visible differences between Galapagos mockingbirds &#8212;
not finches &#8212; that triggered his first ideas about species diversity,
subsequently expanded by Darwin's huge collection of beetles and other
insects. The first edition of his 1859 opus, “On the Origin of Species,”
contains just a single illustration: A stick-like rendering of a tree of
life. It is crudely drawn but visionary. (Cristina Martinez Byvik photo illustration / Union-Tribune)

Charles Darwin's contributions to biology evolved from lessons learned
and species collected during a five-year voyage (1831-36) aboard the HMS
Beagle. It was the visible differences between Galapagos mockingbirds —
not finches — that triggered his first ideas about species diversity,
subsequently expanded by Darwin's huge collection of beetles and other
insects. The first edition of his 1859 opus, “On the Origin of Species,”
contains just a single illustration: A stick-like rendering of a tree of
life. It is crudely drawn but visionary. (Cristina Martinez Byvik photo illustration / Union-Tribune)

Taking time
Generally speaking, Darwin assumed evolution to be a long, gradual process. In the 1970s, paleontologists Niles Eldredge and Stephen Jay Gould proposed a popular alternative called “punctuated equilibrium.” Based upon the fossil record, Eldredge and Gould argued that evolution consisted of long periods of nonchange or “stasis” interrupted by brief bursts in which many new species appeared. The cause of the bursts was not known. Natural selection was believed to be a fine-tuning mechanism, not a major force.

Punctuated equilibrium has fewer adherents now, said David Jablonski, a professor of geophysical sciences at the University of Chicago. Critics contend it doesn't accurately or fully reflect life's revealed complexity; that species diverge and emerge on different timetables, each based on different sets of factors and influences.

David Reznick, a professor of biology at UC Riverside, studies evolution rates in natural populations of guppies in Trinidad. He has recorded change rates “on the order of 10,000 to 10 million times faster than what Gould and Eldredge described as punctuations in the fossil record.”

Selective pressures like food and climate are far more powerful than Eldredge, Gould and other scientists had previously thought. They accelerate change. So, too, do random genetic mutations.

“Organisms like ferns sometimes experience a doubling of chromosomes,” said Reznick. “This change can create a barrier to breeding with other members of what had been the same species; such a barrier is the prevailing definition for species. These rare events can, in theory at least, create species in a single step.”

Like evolution, the broad process of speciation is ongoing and inevitable. It's happening now, everywhere. The ultimate question is, what will happen next? In truth, there's no way to know. The past is not prologue.

In the fossil record, mass extinctions have been followed by periods of huge biological diversification: the rise of mammals, for example, after the dinosaurs died out 67 million years ago.

Though it's commonly believed the dinosaurs' disappearance opened new ecological niches for mammals, Mike Arnold, an evolutionary biologist at the University of Georgia, said every mass extinction may not presage a subsequent blossoming of biodiversity.

“We're in the middle of an enormous mass extinction right now, one that could produce opportunities for new life,” Arnold said. “But this time there's a caveat. This mass extinction is human-mediated. It's largely caused by things like destruction of habitat, which may result in irreversible harm in terms of evolutionary diversification.

“Environments are being modified, but maybe not in any way that life can adapt to and evolve in. There's not a lot of opportunity for diversification in a world of concrete.”

Certainly some kind of life will persist, adapt, evolve. That's what life has done for billions of years and what it will likely do for billions more. It just might not be the kinds of life we value.

It just might not be us.

FURTHER READING

“Why Evolution Is True” by Jerry A. Coyne, Viking, 2009. A new and popular examination of evolution by a renowned expert.

“The Reluctant Mr. Darwin” by David Quammen, Norton, 2007. An intimate biography that explores the man and how he devised evolutionary theory.

“How and Why Species Multiply” by Peter R. Grant and B. Rosemary Grant, Princeton University Press, 2007. Charles Darwin studied the finches of the Galapagos Islands; the Grants have made the finches their life's work, using them as a prism through which they explain evolutionary biology.